In most blasting operations, efficient use of explosive energy includes obtaining the desired breakage and movement of ore and rock. It is also becoming increasingly important to minimize the effects of blasting on nearby structures by maintaining close control over ground vibrations produced by the blast. In a multi-hole blasting pattern, it is usually desirable not to have all of the explosives detonate at one time, but to separate the detonation of each hole by at least eight milliseconds in time to control ground vibrations. The separation of the total weight of explosives used in a blast into smaller charges detonated individually in time sequence is achieved by means of delay blasting. Delay blasting normally involves the use of electric or nonelectric delay blasting caps, detonating cord delay connectors or blasting machines of the sequential type.
All presently manufactured electric and nonelectric delay blasting caps have internal delay elements which are based upon the timed burning of pyrotechnic mixtures compressed into metal tubes. The delay timing is achieved by the ignition and burning of the pyrotechnic mixture.
The problem with pyrotechnic delay blasting caps is that, even under the most careful manufacturing conditions, the delay timing of any given delay period is subject to inherent time scatter due to the nature of the burning process. Therefore, the exact detonation time of the blasting cap cannot be controlled with high precision. Because of time scatter, it is possible for pyrotechnic delay blasting caps of two adjoining delay periods to detonate so close together in time that an undesirable level of ground vibration is produced since more than the optimum weight of explosives is detonted at the same time.
The sequential type blasting machines provide controlled timing electric pulses to electric blasting caps. These timing pulses are formed by electronic means and are precise. However, during blasting, circuit wires between the blasting machine and the electric blasting caps must be maintaind intact until the blasting caps receive the firing pulses from the machine. Therefore, it has been found that sequential switches must be used in conjunction with pyrotechnic delay electric blasting caps placed in the boreholes to minimize the premature breaking or shorting of circuit wires. Problems with control of vibrations therefore are the same as with the aforementioned use of pyrotechnic delay electric blasting caps.
Unless the sequential blast is designed to have all caps ignited before the first hole detonates, the possibility for broken or shorted circuit wires is increased. Many sequential blasting patterns do not permit all caps to be ignited before hole detonation begins.
In many cases, sequential blasting machine patterns are designed so that there are only eight milliseconds between detonations. It can be seen that the normal scatter in pyrotechnical delays will result in detonations at less than eight millisecond intervals and will increase the probability of out of sequence detonations. When this occurs, ground vibrations may be increased and rock fragmentation may be poor.
Because pyrotechnic delay blasting caps must be used with sequential blasting machines, problems with vibration control and rock fragmentations are the same as with the aforementioned use of delay electric blasting caps.
As explained previously, standard delay blasting involves detonating individual explosive columns at predetermined time intervals. During this process, boreholes that detonate at later delay intervals are subjected to shock and gas pressures generated from the detonation of explosives in adjoining boreholes. Blasting caps are required to withstand these pressures and must function properly at the desired delay interval.
The component parts of an electric blasting system include the blasting machine, firing line, connecting wires, and electric blasting caps.
Electric blasting caps are commonly fired from capacitor discharge type blasting machines. These power sources utilize an energy storage capacitor that is charged to a high voltage such as 450 VDC. Upon activation of a firing switch, the energy is released to the blasting caps through a firing line and connecting wires. Low resistance, heavy gauge cooper firing lines and connecting wires are commonly used to minimize energy losses.
Blasting circuits are laid out in series, parallel, or parallel series combinations to permit efficient use of available electrical energy. To assure that the energy is distributed properly, blasting personnel are required to optimize the blasting circuit design by performing energy calculations, which often become difficult and complex. The resistance balancing of parallel branches is also necessary for optimum energy distribution. In the event that the available energy is not distributed properly, and a blasting cap fails to fire because of insufficient current, undetonated explosives will remain in the muckpile resulting in a very hazardous condition.
Many mining and construction companies have difficulty in hiring qualified blasters, and in many cases the turnover of personnel is very high. The frequent training of new blasters, although very important, becomes very costly and time consuming. Therefore, simplification or electric blasting would be advantageous from both a training and the aforementioned safety standpoints.
The high voltage from a standard blasting machine poses either a possible shock hazard condition to blasting personnel or a problem of current leakage from damaged insulation or bare wire connections. A lower voltage electric blasting system would not present a shock hazard, and would be far less susceptible to current leakage, thus, reducing the possibility of misfires.
Electric blasting caps can be fired from a 11/2 volt flashlight cell. It would be desirable to increase this voltage requirement to reduce the susceptibility of the cap to be prematurely initiated by extraneous electricity.
In summary, the need for precise delay timing can be clearly justified by improving rock fragmentation and reducing undesirable levels of ground vibrations. Also, improving the safety of electric blasting systems is a continuing goal for companies associated with explosives. Reliability, susceptibility to extraneous electricity and simplification of firing systems are all vital areas for safety improvement considerations.